In recent years, in the field of machine tools, there is an increasing trend for a compound machine tool to be capable of carrying out various machining processes, such as turning, drilling, milling, etc., with a wide variety of tools, such as cutting tools, drills, milling cutters, etc., the tools being provided in tool rests in a changeable manner, in order to allow workpieces having various complex shapes to be automatically machined. Also, in a lathe, such as a numerically controlled (NC) lathe, configured as such a compound machine tool (hereinafter generally referred to as an automatic lathe), taking into consideration of reduction in machining time, various multi-function type automatic lathes have been proposed, in which one or more spindles and one or more tool rests, respectively operatable along control axes different from each other, are collectively mounted on a single lathe bed, so as to make it possible to perform different machining processes (e.g., outer-diameter turning and boring) simultaneously on a same workpiece, or to perform simultaneous machining processes on different workpieces.
As a tool rest installed in such a multi-function type automatic lathe, a turret tool rest has been well known. The turret tool rest includes a turret provided with a plurality of tool-mount portions at respective indexing angles in a circumferential direction, and turning tools, such as cutting tools, and rotating tools, such as milling cutters, can be selectively fitted to desired tool-mount portions of the turret. Such a turret tool rest is generally provided with an index-drive mechanism for driving the turret in a turning indexing motion (i.e., a tool selecting operation) and with a rotation-drive mechanism for driving the rotating tool in a rotating motion (i.e., a machining operation) independently of each other. However, the turret tool rest may have a mechanical configuration in which, due to the fact that rotating tool remains operatively connected to the suspended rotation-drive mechanism during the turning indexing motion of the turret, the rotating tool rotates, in a passive manner, in accordance with the turning indexing motion. Also, in this mechanical configuration, the relationship between the rotation speed of the turret and the rotation speed of the rotating tool often may not represent an integer multiple relative to each other, depending on the ratio of rotation speed in a gear train provided between the rotating tool and the rotation-drive mechanism.
A rotating tool, such as a hob for gear cutting or a polygon cutter for polygon machining, which executes a cutting process while rotating a workpiece at a predetermined speed, can be fitted to the above-described turret tool rest. In this connection, in the case where the turret tool rest has such a mechanical configuration that the relationship between the rotation speed of the turret and the rotation speed of the rotating tool represents a non-integer multiple as described above, if the turret is turned by one rotation (360°) or more as a result of accumulation of turning indexing motions in the same direction during a procedure that a rotating tool finishes a cutting process on one workpiece at an indexed position and is thereafter disposed again at the indexed position for the same cutting process on the next other workpiece, the positional deviation of the edge of the rotating tool (i.e., the phase shift in the rotating motion) occurs during the two-time location of the tool at the indexed position. As a result, in the workpieces subjected to the cutting process by a hob or a polygon cutter, the position (or the coordinate) of the cut portion (e.g., a tooth of a gear or a lateral face of a polygon bar) of each workpiece upon completion of the cutting process may be differently located, for a two-time cutting process. If a positional deviation occurs, for every cutting process, in the cut portion of a workpiece upon completion of the cutting process, problems may arise wherein, when a secondary machining process, such as a milling process, is performed after the cutting process, a machined position (or a coordinate) on the workpiece may be shifted relative to the former cut portion (e.g., the tooth of the gear or the lateral face of the polygon bar), and thereby an intended product may not be obtained.
In order to eliminate the above problems, a countermeasure has been proposed wherein, when various tools used in a machining program of a workpiece are fitted to a turret tool rest, the tools are arranged in the order of machining steps in a normal rotating direction (e.g., in a clockwise direction) of the turret, and after the machining program is completed for one workpiece, a tool selecting operation is controlled so as to turn the turret in a reverse rotating direction for the selection of the first tool as a preparatory step for the next machining program. According to this measure, the turret does not turn by one rotation or more as the cumulative sum of the turning indexing motions in the same direction. In this measure, however, if a plurality of machining steps using the same tool are contained in the machining program, it is necessary to turn the turret in the reverse rotating direction even halfway in the machining process, under the prerequisite such that the cumulative sum of the turning indexing motions of the turret should not be one rotation or more. As a result, the machining program becomes complicated and may increase a load on an operator. In addition, even if the positional correlation between a currently selected tool being currently used and a next designated tool to be subsequently used is such that the next designated tool can be selected by a shortcut rotation of 180° or less by turning the turret in the normal rotating direction, there may arise a case wherein it is necessary to perform a roundabout rotation of more than 180° by turning the turret in the reverse rotating direction. Thus, the cycle time of the machining program may be unnecessarily increased.